ooni-probe-cli/internal/tutorial/measurex/chapter11

Chapter XI: Measuring a URL

This program shows how to measure an HTTP/HTTPS URL. We are going to call an API whose implementation is basically the same code we have seen in the previous chapter, to obtain an URL measurement in a more compact way. (As an historical note, the API we are going to call has indeed been written as a refactoring of the code we introduced in the previous chapter.)

(This file is auto-generated. Do not edit it directly! To apply changes you need to modify ./internal/tutorial/measurex/chapter11/main.go.)

main.go

The beginning of the program is much simpler. We have removed out custom measurement type. We are now going to use the URLMeasurement type (go doc ./internal/measurex.URLMeasurement), which as the same fields of measurement in chapter10 plus some extra fields that we'll examine in a later chapter.

package main

import (
	"context"
	"encoding/json"
	"flag"
	"fmt"
	"time"

	"github.com/ooni/probe-cli/v3/internal/measurex"
	"github.com/ooni/probe-cli/v3/internal/runtimex"
)

func print(v interface{}) {
	data, err := json.Marshal(v)
	runtimex.PanicOnError(err, "json.Marshal failed")
	fmt.Printf("%s\n", string(data))
}

func main() {
	URL := flag.String("url", "https://www.google.com/", "URL to fetch")
	timeout := flag.Duration("timeout", 60*time.Second, "timeout to use")
	flag.Parse()
	ctx, cancel := context.WithTimeout(context.Background(), *timeout)
	defer cancel()

We create a measurer, cookies, and headers like we saw in the previous chapter.

	mx := measurex.NewMeasurerWithDefaultSettings()
	cookies := measurex.NewCookieJar()
	headers := measurex.NewHTTPRequestHeaderForMeasuring()

Then we call MeasureURL. This function's implementation is in ./internal/measurex/measurer.go and is pretty much a refactoring of the code in chapter10.

The arguments are:

• the context as usual

• the unparsed URL to measure

• the headers we want to use

• a jar for cookies

	m, err := mx.MeasureURL(ctx, *URL, headers, cookies)

The return value is either an URLMeasurement or an error. The error happens, for example, if the input URL scheme is not "http" or "https" (which we handled by panicking in chapter11).

Now, rather than panicking inside MeasureURL, we return the error to the caller and we panic here on main using the PanicOnError function.

	runtimex.PanicOnError(err, "mx.MeasureURL failed")
	print(m)
}

Running the example program

Let us perform a vanilla run first:

go run -race ./internal/tutorial/measurex/chapter11

Take a look at the JSON output and compare it with:

go run -race ./internal/tutorial/measurex/chapter10 -url https://www.google.com

(which is basically forcing chapter10 to run with the the default URL we use in this chapter).

Can you explain why we are able to measure more endpoints in this chapter by checking the implementation of MeasureURL and compare it to the code written in chapter10?

Now run:

go run -race ./internal/tutorial/measurex/chapter11 -url https://google.com

Do you see the opportunity there for following redirections? :^).

Conclusion

We have introduced MeasureURL, the top-level API for measuring a single URL.